JPS6026809B2 - Method for manufacturing high-strength steel pipes with excellent low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-strength steel pipes with excellent low-temperature toughness, using steel of a specific composition that exhibits extremely little deterioration of toughness due to strain aging as the material for the steel pipes, and the rolling conditions and subsequent By appropriately controlling the heat treatment, the aim is to improve the strength without deteriorating the grade at low temperatures.

In recent years, the development of energy resources such as oil and natural gas has been carried out over a wide range of areas, even in cold regions, and as a result, many plans have been made to construct pipelines to transport these resources, and some are already being implemented.

The steel pipes used in these pipelines are gradually becoming larger in diameter, and along with this, there has been a demand for higher tensile strength of the steel pipes. Furthermore, since steel pipes for pipelines in cold regions are installed at low temperatures, there are extremely strict requirements regarding their toughness. The raw material for this kind of high-tensile steel pipe for pipelines is often as-rolled steel plate manufactured by controlled rolling, and currently X7 reinforcement steel pipes are manufactured using this controlled rolling material. There is.

However, although the plan is to use steel pipes that have the high strength of X8 female suction and good low-temperature balling properties, if the above-mentioned rolled material is used, it cannot be expected that the strength will exceed X70 class. X7
Currently, the dimensions that can be manufactured for government-treated steel pipes are limited due to pipe manufacturing capacity. Furthermore, in recent years M○, Ni,
The price of special alloying elements such as V has increased significantly, and the manufacturing cost of X7 female side steel pipes manufactured from controlled rolling materials has also become considerably higher than in the past. Considering the saving of alloying elements and the expansion of manufacturable dimensions, it is desirable to take measures to increase the strength after forming the steel pipe as a manufacturing method for high-strength steel pipes. Although this is a promising method, this treatment requires large-scale equipment and considerably increases the cost of manufacturing steel pipes.

On the other hand, so-called strain aging treatment, which is an aging treatment performed after steel pipe forming, is an effective means for increasing the strength of steel pipes, but it has traditionally been considered undesirable because strain aging generally deteriorates the low-temperature sardonic properties of the material. It had been. However, in this regard, the inventors have conducted numerous studies on the effects of cold working and water aging on the low-temperature toughness of steel pipes manufactured from many materials with different chemical compositions and rolling methods. As a result of repeated experiments and studies, we have found that materials with a specific range of chemical composition and good low-temperature rolling properties produced through appropriately controlled rolling do not suffer from deterioration in low-temperature plowing properties due to cold working and subsequent strain aging. It was found that this is much smaller than previously thought, and that by appropriately combining the amount of cold work and aging conditions, it is possible to increase the strength while maintaining good low-temperature plowability.

This invention is based on the above findings.

That is, the present invention provides C, 0.18% or less by weight.
50% or less Si, 0.50-20% Mh, 0.03
Contains 0% or less P, 0.006% or less S, 0-15% or less Nb, and 0.060% or less AI, and optionally 0.15% or less V or Ti, 0. 50
% or less Mo, 0.50% or less Cu, 3.0% or less Ni, 0.60% or less C, 0.040% or less RE
A steel slab containing one or more selected from M and 0.010% or less of Ca, with the remainder substantially having a composition of Fe, is heated at a heating temperature of 118,000 or less and 950qo
Rolling reduction rate: 65-95%, final rolling finish temperature: 750
The steel pipe is made of a steel plate that is rolled under the conditions of ~550:0, and the maximum value of the total separation length per fracture surface 1 in the Charpy impact test using a 2-side V-notch piece is 18 sides or more through the test temperature. Formed into this steel pipe with 100
This is a method for manufacturing high-strength steel pipes with excellent low-temperature toughness, which comprises aging treatment in a temperature range of ~400 oo. Hereinafter, some of the experimental results that led to this invention will be listed, and the invention will be specifically explained based on them.

Conventionally, control-rolled materials have a significantly low ductile-brittle transition temperature in impact tests, and this is due to grain refinement by controlled rolling and rolling at Q+y castle (1
00) [011] Separation is generated as the orientation develops, and it is known that the amount of separation generated has a particularly large effect on the transition temperature.

On the other hand, it is well known that when a material is subjected to strain aging, its low-temperature toughness deteriorates due to an increase in descending stress due to strain aging and plastic flow stress at low strain levels.

Therefore, until now, it has not been considered to apply strain aging treatment to obtain good low-temperature plowability and strength as the object of this invention. However, according to experiments conducted by the inventors on this point, in an impact test using a two-legged V-notch Sharpie specimen subjected to strong controlled rolling, it was found that "throughout the test temperature for each separation length per fracture surface 1 of the specimen" For materials with a maximum value of 18 or higher, the ductile-brittle transition temperature is dominated by the temperature dependence of the amount of separation generated, and this temperature dependence of the amount of separation generated is not significantly affected by strain aging, so Compared to materials with a maximum separation length of less than 18 sides, as shown in Table 1, the ductile-brittle transition temperature (vTrs) due to strain aging in the Charpy impact test is
) was found to be very small.

The results shown in Figure 1 are obtained by using a rolled material with an eave of X60 to X7 as the steel pipe material, which was also made by the UOE method, expanded at a pipe expansion rate of 1.0%, and then aged for 25,000 mm and 15 minutes. This is about something that has been subjected to.

As is clear from the figure, when the maximum value of the total separation length is 18 or more, the difference ΔvT in the vT values after and before the statute of limitations is extremely small. On the other hand, looking at the relationship between the fracture surface and the impact value when such a strong controlled rolled material is subjected to a Charpy impact test, it is found that in the temperature range where a 100% ductile fracture surface occurs, the lower the temperature, the more separations occur. Along with this, the absorbed energy decreases, and 1
Absorbed energy at the lowest temperature at 00% ductile fracture, so-called Cv, shows a low value. Therefore, at low temperatures, there is a risk that a fracture called unstable ductile fracture may occur, which propagates at high speed even though the fracture surface is 100% ductile. Therefore, it has been difficult to use steel pipes formed from controlled-rolled materials, which may cause unstable integrity fractures, as steel pipes for large-diameter gas pipelines that require good toughness even at low design temperatures, i.e., even at fairly low temperatures. was shown. However, regarding this point, even if the Cv side is small,
It has been found that there is no problem in using controlled rolled materials for gas pipelines as long as the temperature at which this value is shown is sufficiently low and the material has a sufficiently high absorbed energy at the design temperature and therefore at a fairly low temperature. Therefore, the inventors carried out intensive research into the development of a steel pipe material that would have high absorption energy in low-temperature castles even if a large number of separations occurred.
Using a slab manufactured by controlling the sulfide morphology by controlling the amount of sulfide to 0.006% or less and adding rare elements (REM), Ca, Mg, or Zr as necessary, the rolling stage is particularly By specifying the conditions for controlled rolling according to the present invention, a material can be obtained in which the maximum value of the total separation length per fracture surface 1 in the Charpy test of controlled rolled material is 18 skins or more, and for this material, its absorbed energy As shown in Fig. 2, the deterioration due to strain aging of the material was found to be much smaller than that of materials whose maximum total separation length per cross section was less than 18 sides.

The results shown in Figure 2 show the absorbed energy of steel pipes that were made and aged under the same conditions as explained in Figure 1 above, using steel pipe materials of standard X60 to X7 before aging. , is shown in comparison with that before aging treatment, and from the same figure, when the maximum value of the total separation length is 18 or more,
vE-2 after and before the statute of limitations.

It can be seen that there is almost no difference ΔvE-2o. Next, according to the present invention, a steel pipe with an outer diameter of 1219 sails and a wall thickness of 15.2 ribs was manufactured using a slab melted to the chemical composition shown below and under the rolling and pipe forming conditions shown below.
Figures 3a and b show the results of examining the tensile strength (T.S), yield strength (Y.S), vE law, and vTrs when the aging temperature was variously changed for this steel pipe.
and c.

o Chemical composition (wt%) C: 0.06, Si: 0.25,
Mh: 1.66, P: 0.017, S: 0.002, N
b: 0.041, AI: 0.034, V: 0.030,
Cu: 0.22, Ni: 0.22o Rolling conditions Heating temperature: 1150qo, Reduction rate at 95000 or less:
75%, Final finishing temperature: 700 ooo Pipe forming processing conditions Pipe forming method: UOE method, Pipe expansion ratio: 02%, Mouth 4%, △6% In addition, the total separation length per fracture surface 1 in the 2 rib V notch Charpy test The maximum value was on the 33 side.

UO formed from a raw steel plate manufactured according to this invention
It can be seen that in E steel pipes, excellent strength can be obtained without deteriorating low-temperature toughness by setting appropriate aging conditions. In particular, by appropriately combining it with the tube expansion rate, a high strength of x8 strength can be obtained. Next, the reason why the composition range of the steel pipe material is limited as described above in this invention is as follows.

C is necessary to increase the strength of steel, but if it exceeds 0.18%, low-temperature plowability and weldability are significantly deteriorated.

Si is effective for deoxidizing steel and increasing its strength, but 0
．． If it exceeds 50%, low temperature toughness will be significantly deteriorated.

Mn is effective for increasing the strength of steel and improving low-temperature toughness, but if it is less than 0.50%, it has little effect.2.
If it exceeds 0%, weld cracks will occur frequently.

P is an element that inevitably exists in steel as an impurity,
If it exceeds 0.030%, low-temperature ballability will be significantly deteriorated, so the content is set to 0.030% or less. Like P, S is an unavoidable impurity element, and it is desirable to keep it as low as possible.
If it exceeds 0.006%, the impact absorption energy will be significantly reduced even if the sulfide morphology is controlled, so 0.006%
I've included it below.

Nb is added as an essential element to increase the strength of steel and significantly increase the effect of controlled rolling, but at 0.15
%, the low-temperature properties of the weld metal will be significantly deteriorated, so the upper limit was set at 0.15%.

Although AI is effective for deoxidizing steel and refining grains, when it exceeds 0.060%, surface defects and internal defects occur frequently in steel pipes. V is sometimes added to increase the strength of steel, but if it exceeds 0.15%, it deteriorates low temperature toughness.

Ti is sometimes added to increase the strength of steel and refine grains, but if it exceeds 0.15%, it significantly deteriorates low-temperature graininess. Cu is sometimes added to increase the strength of steel, but if it exceeds 0.50%, surface defects will occur frequently in the steel pipe.

Ni is sometimes added to increase strength and improve low-temperature rutting properties, but it is expensive, and 3.0% Ni is sufficient for improving low-temperature toughness of steel pipes for gas pipelines. The upper limit was 0%.

Cr is sometimes added to increase the strength of steel, but 0
．． If it exceeds 60%, low temperature toughness will deteriorate.

The above-mentioned components from V to C are equivalent components that particularly contribute to improving the strength. Both REM and Ca are effective components for controlling the morphology of sulfides, but if they are added in large amounts, internal defects in steel pipes will occur frequently, so each RE
The upper limits were M: 0.040% and Ca: 0.010%.

Next, the reason why the slab heating temperature was set to 1180 qo or less is because if the slab is heated to a temperature exceeding this, good low-temperature properties cannot be obtained. The rolling reduction rate at 950qo or less is 65~
The reason why it is set at 95% is because if the rolling reduction is less than 65%, the maximum value of the total separation per fracture surface in the Shall Pey impact test using a two-sided V/cchi specimen cannot be increased to 18 legs or more. Not only impossible, but also sufficiently low ductility
This is because a brittle transition temperature cannot be obtained, and on the other hand, if it exceeds 95%, the strain in the longitudinal direction and middle direction of the plate becomes large, and it cannot be used as a product.

The reason for setting the final finishing knitting degree to 750 to 550°C is that when the finishing temperature exceeds 750 qo, the maximum value for each separation length per fracture surface 1 cannot be made more than 18 sides, and the ductility-brittleness is sufficiently low. This is because if the transition temperature cannot be obtained, and on the other hand, the temperature is less than 550q0, the deformation resistance of the steel sheet becomes significantly high, making normal rolling impossible.

The heating temperature during aging treatment was set at 100 to 400°C because
At temperatures below 100°C, sufficient age hardening does not occur;
This is because if the temperature exceeds C, the softening due to over-aging will be significant, and in any case, the effect of increasing the strength will be poor.

Further, it is effective that the time required for the aging treatment is about 0.5 to 12 minutes.

This is because if the aging treatment time is less than 0.5 minutes, the age hardening will not be sufficient, and if the aging treatment time is 120 minutes, the hardening due to aging will be sufficiently saturated, and if it exceeds this, the strength may decrease due to overaging. It is. For heating during aging, any of an electric path, a gas furnace, and an induction heating device may be used, or a method of heating with a gas burner while rotating the steel pipe may be used.

Examples of the present invention will be described below. Steel pipes were manufactured according to the present invention under the conditions shown in Table 1, and their tensile properties and impact properties were investigated.

The results are also shown in Table 1. For comparison, similar tests were also conducted on steel pipes whose manufacturing conditions were outside the scope of the present invention, and the results are also shown in Table 1. Funai: Funago [Paper 1 to 18 are invented steel pipes, and No.
Nos. 19 to 26 are comparison steel pipes.

No. Nos. 19 and 20 are the invention steel pipe Nos. 1 and 2 when the aging treatment temperature is too high or too low. Compared to No. 9 to No. 11, all have good low-temperature sardine properties but low strength. No
．． In Nos. 21 and 22, the rolling conditions are outside the scope of the present invention, the maximum value of the total separation length is short on the 3 side, and both have low low temperature toughness. No. Nos. 23 and 24 do not contain Nb, which is an essential component, and the rolling conditions are outside the appropriate range, so the controlled rolling effect is poor, and no separation is observed at all. Therefore, the impact properties are low. No. Nos. 25 and 26 also have inferior impact properties because the rolling conditions are outside the appropriate range. On the other hand, the steel pipes (flanks 1 to 18) manufactured according to the present invention all exhibit excellent tensile and impact properties, and not only have excellent low-temperature rutting resistance but also have high strength of X80 class. .

Although the above description has mainly been about the case where the UOE method is used as a pipe manufacturing method, the method of this invention can of course be applied not only to UOH steel pipes but also to full-circle steel pipes and spiral steel pipes. In comparison, it is inexpensive and can easily produce high-strength, high-strength steel pipes, so its industrial value is great.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the maximum amount of separation generated throughout the test temperature in the Charpy test and the amount of change in vT of the material due to strain aging. Figure 2 is the maximum amount of separation generated throughout the test temperature in the Charpy test. Figure 3 a, b and c are graphs showing the relationship between the amount of change in shear py absorbed energy of the material due to strain aging, and the tensile properties and impact when the amount of tube expansion and aging temperature in the UOE process are changed. It is a graph showing changes in characteristics. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. 0.18% or less C, 0.50% or less Si by weight
, 0.50-2.0% Mn, 0.030% or less P,
0.006% or less S, 0.15% or less Nb and 0
．． A steel slab containing 0.60% or less Al and the remainder being substantially Fe is heated at a heating temperature of 1180°C or less at 95%
Reduction rate 65-95% below 0℃, final finishing temperature 750℃
A steel pipe is formed from a material steel plate which is rolled under conditions of ~550°C and has a maximum separation length per cm^2 of fracture surface in a sharpie impact test using a 2mm V-notch specimen over the test temperature of 18mm or more, A method for producing a high-strength steel pipe with excellent low-temperature toughness, which comprises then subjecting the steel pipe to an aging treatment at a temperature range of 100 to 400°C. 2 0.18% or less C, 0.50% or less Si by weight
, 0.50-2.0% Mn, 0.030% or less P,
0.006% or less S, 0.15% or less Nb and 0
．． 0.60% or less Al, further 0.15% or less V or Ti, 0.50% or less MO, 0.50% or less Cu, 3.0% or less Ni, 0.60% or less Cr
, 0.040% or less of REM, and 0.010% or less of Ca, with the remainder being substantially Fe at a heating temperature of 11%.
Rolled under the conditions of 80℃ or lower, 950℃ or lower pressure rate 65-95%, final finishing temperature 750-550℃, 2m
A steel pipe is formed from a material steel plate in which the maximum separation length per 1 cm^2 of the fracture surface in a shear pie impact test using a mV notch specimen is 18 mm or more throughout the test temperature, and then this steel pipe is heated to a temperature of 100 to 400 °C. A method for manufacturing high-strength steel pipes with excellent low-temperature toughness, which is characterized by subjecting them to aging treatment over a range of conditions.